1. Field of the Invention
The present invention relates to a radio communication system, which includes a radio terminal and a plurality of base stations. In the radio communication system, the base stations communicate with the radio terminal by connecting radio links with mutually different frequencies between base stations, and relates to the radio terminal, the base stations, control equipment and a communication method, which are used in the radio communication system.
2. Description of the Related Art
In recent years, there has been an increase in demand for high-speed data communications in the field of radio communications such as portable telephone communications. Thus, an International Mobile Telecommunication-2000 (IMT-2000) system has been introduced to enable high-speed communications with a maximum communication speed of 2 Mbps. In the IMT-2000 system, an IMT-2000/Time Division Duplex (TDD) has been standardized. In the IMT-2000/TDD, a length of one frame is 10 ms. This one frame is divided into fifteen time slots. Each time slot is assigned to an uplink time slot used for uplink communication from a radio terminal to a base station, or to a downlink time slot used for downlink communication from the base station to the radio terminal. Accordingly, in the IMT-2000/TDD, a ratio of uplink traffic to downlink traffic can be freely changed by controlling this time slot assignment. Moreover, the IMT-2000 system adopts Code Division Multiple Access (CDMA) to enable transmission/reception of a plurality of signals in the same time slot.
In the radio communication system which uses such an IMT-2000/TDD, as shown in FIGS. 1 and 2, a radio terminal 210 and base stations 220, 230 communicate by constituting one frame of fifteen time slots numbered 1 to 15 and assigning the time slots to uplink and downlink time slots. In FIGS. 1 and 2, an abscissa indicates a time axis. The drawings show transmission/reception of signals carried out by the radio terminal 210 and the base stations 220, 230 in relation to the time axis.
In FIG. 1, first, the base station 220 transmits a control signal A310 in a time slot No. 1 of a frame No. 1. In the same time slot No. 1 of the frame No. 1, the base station 230 transmits a control signal B311 with a frequency different from that used by the base station 220. Thus, in FIG. 1, the base stations 220, 230 both transmit control signals in the time slot No. 1 of each frame. Therefore, the time slots for transmitting the control signals are synchronized.
At this time, the radio terminal 210 receives the control signal A310 from the base station 220, which is in communication with the radio terminal 210. Then, in a time slot No. 5 of the frame No. 1, the radio terminal 210 transmits a communication signal C320 including data to the base station 220. The radio terminal 210 determines transmission power for transmitting the communication signal C320 based on the reception power of the control signal A310 received in the time slot No. 1. Then, the base station 220 transmits a communication signal D330 to the radio terminal 210 in a time slot No. 7 of the frame No. 1.
Subsequently, in a time slot No. 1 of a frame No. 2, the base station 220 transmits the control signal A310, and the base station 230 transmits the control signal B311. At this time, the radio 210 carries out a handover, if the radio terminal 210 has moved from a cell covered by the base station 220 to a cell covered by the base station 230. Specifically, if a quality of a radio link deteriorates while the radio terminal 210 is communicating with the base station 220 by connecting the radio link, the radio terminal 210 carries out the handover, which comprises switching from the radio link with the base station 220 to a radio link with the other base station 230. Thus, the radio terminal 210 receives the control signal B311 from the base station 230.
In FIG. 2, first, the base station 220 transmits the control signal A310 in the time slot No. 1 of the frame No. 1. At this time, the radio terminal 210 receives the control signal A310 from the base station 220, which is in communication with the radio terminal 210. Then, in the time slot No. 5 of the frame No. 1, the radio terminal 210 transmits the communication signal C320 including data to the base station 220. The radio terminal 210 determines the transmission power for transmitting the communication signal C320 based on the reception power of the control signal A310 received in the time slot No. 1.
Then, in the time slot No. 7 of the frame No. 1, the base station 230 transmits the control signal B311 with a frequency different from that used by the base station 220. In the same time slot No. 7 of the frame No. 1, the base station 220 transmits the communication signal D330 to the radio terminal 210. Thus, in FIG. 2, the base stations 220, 230 transmit the control signals in the different time slots of each frame. At this time, the radio terminal 210 receives the communication signal D330 from the base station 220, which is in communication with the radio terminal 210.
Subsequently, similarly, in the time slot No. 1 of the frame No. 2, the base station 220 transmits the control signal A310. In the time slot No. 5 of the frame No. 2, the radio terminal 210 transmits the communication signal C320. Then, in a time slot No. 7 of the frame No. 2, the base station 230 transmits the control signal B311 to the radio terminal 210, and the base station 220 transmits the communication signal D330 to the radio terminal 210. At this time, if the radio terminal 210 has moved to the cell covered by the base station 230, the base station 230 becomes the base station to which the radio terminal 210 connects a new radio link according to the handover. Thus, the radio terminal 210 receives the control signal B311 from the base station 230.
However, in the case of the conventional communication method where the plurality of base stations 220, 230 transmit the control signals in the same time slot of each frame as shown in FIG. 1, there are the following problems. Before the execution of the handover, the radio terminal 210 receives the control signal B311 transmitted from the base station 230 as a handover destination, where the new radio link is connected, and measures the quality of the control signal. On the other hand, the radio terminal 210 determines the transmission power for transmitting the communication signal C320 based on the reception power of the control signal A310 received from the base station 220, which is in communication with the radio terminal 210 and controls the transmission power.
However, as in the frame No. 1, when the radio terminal 210 receives the control signal A310 from the base station 220, which is in communication with the radio terminal 210, the radio terminal 210 cannot receive the control signal B311 from a base station other than the base station 220, which is in communication with the radio terminal 210, i.e., the base station 230. Therefore the radio terminal 210 cannot receive the control signal, which is necessary for the handover. On the other hand, when the radio terminal 210 receives the control signal B311 from a base station other than the base station 220, which is in communication with the radio terminal 210, i.e., the base station 230, the radio terminal 210 cannot receive the control signal A310 from the base station 220, which is in communication with the radio terminal 210. Accordingly, the radio terminal 210 cannot control the transmission power when the radio terminal 210 transmits the communication signal C320, and communication capacity is reduced. Therefore, it is impossible to satisfactorily communicate.
Further, in the case of the conventional communication method where the plurality of base stations 220, 230 transmit the control signals in the different time slots of each frame as shown in FIG. 2, there are the following problems. As in the time slot No. 7 of the frame No. 1, when the radio terminal 210 receives the control signal D330 from the base station 220, which is in communication with the radio terminal 210, the radio terminal 210 cannot receive the control signal B311 from a base station other than the base station 220, which is in communication with the radio terminal 210, i.e., the base station 230. Therefore, the radio terminal 210 cannot receive the control signal necessary for the handover.
On the other hand, when the radio terminal 210 receives the control signal B311 transmitted from the base station 230 in the same time slot for the communication signal C320 from the radio terminal 210 to the base station 220 which are in communication with the radio terminal 210, the radio terminal 210 cannot transmit the communication signal C320 to the base station 220, which is in communication with the radio terminal 210. When the radio terminal 210 receives the control signal B311 transmitted from the base station 230 in the same time slot as that used for transmission/reception of the communication signal D330, the radio terminal 210 cannot receive the communication signal D330 from the base station 220, which is in communication with the radio terminal 210. Therefore, it is impossible to satisfactorily communicate between the radio terminal 210 and the base station 220 which are in communication with each other.